Hematopoietic stem cells (HSC) can self-renew and give rise to all the cells of the blood and the immune system through successive stages of progenitor cells that are progressively more restricted in their lineage potential. Although a multitude of cytokines that affect HSC are known, the exact mechanisms that regulate self renewal, differentiation and quiescence as well as homeostatic responses of the hematopoietic stem and progenitor cell (HSPC) compartment are not well understood. As hematopoiesis shows extensive mouse strain-dependent, genetically determined variation, elucidation of the mechanisms underlying this variation may be powerful approach to dissect the multiple pathways involved in the regulation of self-renewal, differentiation and proliferation of HSPCs. The goal of the proposal for the first funding period of this grant was to elucidate the mechanisms underlying quantitative genetic variation in the function and size of the HSC compartment. The hypothesis was that transforming growth factor-beta2 (TGF-b2) played a critical role in this process, and that at least one quantitative trait locus (QTL) was located in the telomeric region of chr. 4 and regulated the function of HSPCs through its effect on TGF-b2 signaling. During this funding period we have shown that TGF-b2 is a positive regulator of the responsiveness of HSPCs to the cytokine, Flt3 ligand (Flt3L), an effect that is not shared by other TGF-b isoforms, which have only antiproliferative effects. TGF-b2 requires the presence of serum factors, which ultimately switches the biological effect of TGF-b2 from generic inhibition of proliferation to specific enhancement of Flt3L responsiveness. Both the activity of these serum factors, and the responsiveness of HSCPs to the combined effect of serum and TGF-b2 increase with age. Furthermore, a strong QTL in the telomeric region of chr.4, Tb2r1, regulates the activity of this regulatory mechanism. We identified a most likely candidate gene for this QTL, and, through this observation, could elucidate the nature of the serum factors involved. These findings have obvious implications for acute myeloblastic leukemia, as in up to 30 % of cases; a constitutively active Flt3 receptor plays a role in pathogenesis. Furthermore, TGF-b2 may be an important and genetically regulated determinant of the toxicity of cell cycle-specific chemotherapeutic agents in humans. The goal of this competitive renewal is one, to examine the function of the gene underlying Tb2r1, two, to define the physiological and potential biomedical significance of TGF-b2 signaling in HSPCs, and three, to elucidate the molecular mechanisms underlying the interaction between serum factors, TGF- b2, and Flt3L.